To scratch the back of one's head is really iterpreted in traditional Russian discourse as a sign of misunderstanding / not understanding something, or that of a long and fruitless thinking over smth. There are also idiomatic expressions чесать в затылке/затылок and therefore the expression of "задний ум"
http://dic.academic.ru/dic.nsf/dic_synonims/267751/%D1%87%D0%B5%D1%81%D0%B0%D1%82%D1%8C
This expression has slightly negative overtones, although there is another dictionary giving it as an only correlate in Russian together with some more examples (in old orthography)
http://dic.academic.ru/dic.nsf/michelson_old/8109/%D0%A0%D1%83%D1%81%D1%81%D0%BA%D0%B8%D0%B9
лестничное остроумие is calqued French l'esprit d'escalier with the same meaning of 'starecase wit'
So the strategies are either to use English calque быть умным после события, or to use a neutral saying
осознать/обдумать событие пост-фактум
or to use minimal syntax with prefixes:
додумать (to rethink, to have afterthouhts)
переосмыслить (to rethink)